May 2010

May 25, 2010

For those of you who
just tuned in, this series of posts focuses on recently published articles in
cell biology.Click herefor details.

I recently attended a Keystone conference on cilia where
there was discussion about how actin and membrane trafficking contribute to
ciliogenesis.So, papers on this topic
have been catching my eye lately.One
paper I noticed recently in The Journal of Cell Science was an
ultrastructural analysis of the ciliary pocket; a membrane invagination in
which the base of some cilia is rooted.Many of the studies analyzing ciliogenesis use as a starting point a
spectacular TEM study published almost 50 years ago in the JCB (Sorokin, 1962), which I described recently.Now, Molla-Herman et al. analyzed primary
cilia in various cell lines and in mouse tissue using TEM, SEM and
immunofluorescence. The authors find the ciliary pocket present in at least a
subset of cilia in all of the systems they analyzed.Moreover, the pocket was present in both
motile and primary cilia despite the fact that these two types of cilia form in
different ways.Associated with these
pockets are actin filaments and clathrin-coated pits, which were endocytically
active.The authors speculate that actin
filaments could be involved in positioning the cilium or in transmission of
mechanical stress. Endocytosis in this compartment could be important for
removal or recycling of membranes and membrane proteins.As the authors point out, the ciliary pocket
and its associated actin filaments and clathrin-coated pits is reminiscent of
the flagellar pocket of Trypanosomatids.For a discussion of the membrane biology surrounding the formation of
cilia, the flagellar pocket, cytokinesis and the immunological synapse, check
out the excellent review from Gillian
Griffiths that we published recently and also the interview with her in our biobytes
podcast.

During an infection, immune cells, such as leukocytes,
migrate to their site of action by chemotaxis.Well-known regulators of chemotaxis include G-protein coupled receptors
and Rho family GTPases.Reporting in Nature Immunology, Colvin et al. identify a new pathway in these cells that controls their migration in response to chemokines.To identify new regulators of leukocyte
chemotaxis, the authors performed an RNAi screen. Unexpectedly, their screen
pulled out members of the synaptotagmin family - membrane proteins that promote
calcium-dependent membrane-fusion events such as synaptic vesicle release or
lysosome exocytosis.They also
determined that two Rabs that are involved in vesicle fusion and exocytosis
regulate leukocyte chemotaxis.Interestingly, these proteins do not work through the signaling pathways
known to be important for chemotaxis.So
some kind of calcium triggered, lysosome fusion event promotes chemotaxis, but
how?The authors show that the
synaptotagmins are important for detachment of the uropod, the structure at the
trailing edge of the cell that adheres to the substratum.Although many details need to be worked out,
the identification of a new pathway in a well-studied process is always
exciting.Plus, in this case, a lot is
known about synaptotagmins and their interaction partners etc. so the next
steps could be relatively easy to uncover.

Moving on to developmental patterning in the plant world, Carlsbecker et al.in a recent issue of Nature examine the signaling that
underlies formation of xylem, the tissue in plants responsible for conducting
water from the root to the shoot.They
show that patterning of this tissue in Arabidopsis
requires a two-way signaling process with the components moving from one cell
type to another.It was known that the
transcription factor SHORT ROOT (SHR) could move between cells: it is produced
in the vascular tissue and moves to the endodermis where it activates the
transcription factor SCARECROW (SCR).The authors show that these two transcription factors need to be present
in the endodermis to promote xylem formation.In the endodermis, these factors induce two microRNAs, which then move
back to the vascular cylinder where they target several transcription factors
of the HD-ZIP III family for degradation.This cell-to-cell communication is crucial for xylem development.What I found particularly cool, is that the
amount/activity of the HD-ZIP III transcription factors determines the tissue
type (when these transcription factors are low, the central xylem or metaxylem
forms, at higher levels, the surrounding layers become protoxylem).So the study implies that the microRNAs may be
creating a gradient of their target to allow the specification of cell
fate.The idea that a microRNA could act
in this way is relatively new.It’s a
nice paper: check it out.

May 18, 2010

Today’s new issue of the JCB is full of exciting papers, so let’s get straight down to a roundup of some of the highlights…

Two papers examine how kinetochores hold on to the growing and shrinking ends of microtubules during mitosis. As the University of Washington’s Trisha Davis tells me in this week’s In Focus, “It’s pretty amazing. Thousands of subunits come off the microtubule yet the kinetochore hangs on. And when subunits are added to the microtubule end, the kinetochore stays bound to the tip.” Davis and her collaborators (Tien et al.) along with Stefan Westermann and colleagues from the IMP in Vienna (Lampert et al.) reveal that the kinetochore accomplishes this through the coordinated activity of two microtubule-binding complexes, Ndc80 and Dam1. Both groups show that Dam1 “tracks” the dynamic tips of microtubules, pulling Ndc80 and the rest of the kinetochore with it. The aurora B kinase can disrupt the interaction between Dam1 and Ndc80, which could be a new mechanism by which the kinase eliminates incorrect kinetochore-microtubule attachments.

Elsewhere, another pair of papers examines how cells orient their mitotic spindles during epithelial cyst morphogenesis. Previous work showed that the rho GTPase Cdc42 aligns the spindle perpendicular to the apical-basal axis of the cell, so that the daughter cells' apical surfaces always end up facing the interior of the cyst, eventually forming a lumen in the center. Qin et al. perform an RNAi screen to identify guanine nucleotide exchange factors that activate Cdc42 in this process, and identify Tuba – a GEF that localizes to the cells’ apical cortex. Meanwhile, Rodriguez-Fraticelli et al. find that the centrosomally-localized Cdc42 GEF Intersectin2 also regulates spindle orientation. The image to the left shows how cysts lacking Intersectin2 (upper panel) form multiple small lumens instead of the single, large lumen formed by control cells (lower panel).

Lee et al. reveal how the ubiquitin ligase Parkin promotes the removal of defective mitochondria, and how this process goes awry in Parkinson’s disease. The researchers found that cells expressing mutant versions of Parkin failed to clear damaged mitochondria. Different Parkin mutants blocked the process at different steps: Parkin mutants lacking ubiquitin ligase activity accumulated defective mitochondria in large, peri-nuclear aggregates. Wild type Parkin cleared these aggregates by ubiquitinating the mitochondria to recruit components of the autophagy machinery leading to the organelles’ degradation. Other Parkin mutants blocked the process at earlier stages, either failing to recognize and bind damaged mitochondria, or failing to transport them along microtubules to the peri-nuclear region. Senior author Tso-Pang Yao explains here how mitochondrial degradation resembles the removal of toxic cytosolic proteins. Lee et al.’s research suggests the two processes may be linked, which is fascinating because mitochondrial dysfunction and protein aggregation are both common features of Parkinson’s disease.

Neurological disease is also caused by mutations in the transcription factor SOX10, which is required for the maintenance of neural crest stem cells and for specifying their differentiation into Schwann and other glial cells. To see if SOX10 continues to function later in development, Finzsch et al. delete the transcription factor from immature Schwann cells after their specification. Schwann cells fail to develop any further in the absence of SOX10 and lose their identity, resulting in numerous peripheral nervous system defects. For example, in wild type sciatic nerves (left), large diameter axons are individually wrapped in thick myelin sheaths. But in mutant nerves (right), a single Schwann cell surrounds multiple large and small axons.

Finally for today, Stramer et al. reveal how bundled microtubule “arms” point Drosophila macrophages in the right direction as they migrate toward wounds and help the cells repel each other when they collide. Our cover this week shows two macrophages bumping into each other in living fly embryos – the cells are spectacularly amenable to in vivo imaging, which allowed Stramer et al. to identify the bundles and determine that they require the microtubule-stabilizing protein CLASP. You can read a summary of the paper here, or you can watch the video below – the latest in our biosights series of video podcasts – where lead author Brian Stramer likens the microtubule bundles to a stiff-arm tackle in American football, allowing colliding cells to fend each other off.

Lots of other great papers in today’s new issue – you can find them all on our table of contents by clicking here.

May 12, 2010

This week marks the
start of a new (and hopefully fairly regular) series of posts on recently
published articles in cell biology.I
will provide some brief insight into why I mention a particular paper but this
is not meant to be a comprehensive resource, nor is it meant to comment on the
strengths or weaknesses of the studies.The goal is to let JCB readers know of recent work from other journals that
I think is noteworthy and to hopefully expand the scope of what people
traditionally think of as cell biology.With this disclaimer, read on….

In the May issue of Nature
Cell Biology, Urban et al. (2010) show that actin filaments in lamellipodia
are not branched.This is quite
surprising as it has been thought for a long time that the Arp2/3 complex
promotes the branching of filaments in these actin-rich structures at the
leading edge of cells.Urban et al.
examine lamellipodia in four different cell types using electron
tomography.This 3-D approach revealed
the presence of overlapping filaments rather than branched ones and indicates
that the previous observations of branched filaments might have been due to
technical artifacts.

In the May 7th issue of Science, Shields et al.
(2010) provide insight into how tumor cells evade the immune system.Tumor cells manipulate their microenvironment
or stroma to help their growth and dissemination.For example, tumor cells use macrophages in
their microenvironment to promote invasion and metastasis. I find this
relationship between cancer cells and the immune system quite fascinating.Now, Shields et al. show that tumors
secreting the cytokine CCL21 appear to create a microenvironment that mimics
the normal lymph node stroma--an environment that promotes immune
tolerance--thereby allowing the tumor cells to evade detection by the immune
system.

Finally, in the May 6th issue of Nature, Craven
et al. (2010) perform pronuclear transfer in human embryos in an interesting
study that I’m sure will be controversial.These investigators are interested in genetic disorders caused by
mutations in mitochondrial genomes that are usually maternally
transmitted.Using abnormal embryos from
a fertility clinic that have multiple nuclei at the one-cell stage, the authors
perform experimental manipulations that result in the formation of a recipient
zygote lacking a nucleus and two donor pronuclei that are surrounded by
membrane but lack mitochondria.They
then fuse the recipient zygote with the two donor cells and follow the
development for 6-8 days.Analysis of
the mitochondrial DNA revealed that it was largely from the recipient
zygote.The authors think that this kind
of an approach could be used eventually to prevent the transmission of
mitochondrial DNA diseases.

May 03, 2010

Invadopodia (the red protrusion that’s breached the green ECM layer on this issue’s cover) are specialized membrane extensions and focused areas of metalloprotease secretion that allow epithelial tumors to invade other tissues. Schoumacher et al. report that the actin, microtubule, and intermediate filament cytoskeletons cooperate to form and extend invadopodia by cancer cells. Actin bundles push out the invadopodia initially, but microtubules and intermediate filaments also become involved as the protrusions grow.

How osteoblasts cope with the large amount of collagen they have to secrete during bone development is the topic of a study by Sohaskey et al. and this issues’ In Focus article.Mice lacking a protein called Osteopotentia (shown here on the right) have smaller and more brittle bones than normal. The team traced the protein to the rough ER where it helps cells expand their endoplasmic reticulum to accommodate increased collagen synthesis during osteoblast maturation.

Alterations in dendritic spine structure and function are believed to contribute to mechanisms of learning and memory.In this issue, Mendez et al. find that expression of N-cadherin, a trans-synaptic cell adhesion molecule, is correlated with activity-mediated spine stabilization.This new mechanism for regulation of spine dynamics is highlighted in a comment by Jyothi Arikkath.

Other features in this issue not to be missed are an Editorial by former JCB Editor Mike Rossner (now Executive Director of RU Press), who discusses challenges facing "open access" to published biomedical research. And in this month’s biobytes podcast, James Nelson discusses membrane functions of a-catenin (Benjamin et al.) and Lesilee Rose explains how Let-99 positions the mitotic spindle during asymmetric division in C. elegans embryos (Krueger et al.).Finally, a fascinating Review in this issue of JCB by Gillian Griffiths and colleagues explores similarities between the immunological synapse and ciliogenesis, which she describes in a conversation with Ben Short.

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